This invention relates to compositions containing polymerizable unsaturated ethers of methylol-melamines. It particularly relates to polymerizable acryloyloxyethoxymethyl-melamines and to an improved process for the preparation of these compounds.
Unsaturated ethers of methylol-melamines, such as acryl- oyoleyxethoxymethyl-melamines, are readily polymerizable to hard films which are useful in coatings and castings. The preparation of these compounds is described in U.S. Pat. No. 3,020,255. In accordance with the process described in this Patent, the compounds are prepared by the transetherification reaction of an unsaturated alcohol and the methyl ethers of methylol-melamines. The unsaturated alcohol may contain other groups attached thereto. So, for example, a suitable unsaturated alcohol is acryloyloxyethanol; i.e., hydroxyethyl acrylate. In the transetherification reaction, the methyl groups in the methyl ethers of methylol-melamines are replaced by acryloyloxyethyl groups. The Patent also discloses the use of acidic catalysts such as, for example, formic, acetic, propionic, maleic, tartaric, trichloracetic, toluenesulfonic phosphoric, hydrochloric, hydrobromic, sulfuric, and nitric acids as catalysts in the transetherification reaction. The process described in the Patent is carried out at a temperature ranging from room temperature to about 75.degree. C., although higher temperatures may be used. The methanol by-product which is formed was preferably removed by conducting the reaction under partial vacuum which facilitated the evaporation and consequent removal of this volatile compound.
However, as reported by Giacobbe, et al. in Macromolecules 4, (No. 5) 630 (1971), both the process described in the U.S. Pat. No. 3,020,255 and the products obtained thereby had serious shortcomings. Methylol-melamines can contain from one to six (1-6) methylol groups attached to the melamine nucleus. Usually, the methylol-melamines contain six (6) methylol groups; i.e., the compound is hexamethylol-melamine and the methyl ether thereof is hexamethoxymethyl-melamine. Giacobbe, et al. found the process of the U.S. Pat. No. 3,020,255 to be useful for the transetherification of only one (1) of the methoxy groups. Using the process, they could not replace more than two (2) of the methoxy groups by an acryloyloxyethoxy group. Furthermore, they found the reaction to be very slow at room temperature, and at elevated temperatures polymerization occurred.
Other shortcomings in the process described in the Patent and the products obtained therefrom include self-polymerization of the product during the reaction and side reactions, particularly the methanolysis of the hydroxyethyl acrylate to produce ethylene glycol and methyl acrylate. Ethylene glycol reacts with the melamine product resulting in undesirable gellation. Methyl acrylate has an objectionable odor and is toxic. For these reasons, its presence in the product is undesirable. The presence of these undesirable materials thus necessitates further purification of the reaction mixture containing the product before it is suitable for use.